Versatility of an intramolecularly hydrogen-bonded 4-(N,N-dimethylamino)benzoate group as a signaling subunit for anion recognition

The versatility of the 4-(N,N-dimethylamino)benzoate (DMAB) group embedded in host as a signaling subunit for anion recognition was elucidated in terms of 1H NMR, CD, and fluorescence studies. Host showed 1:1 complexation with monovalent anions and stepwise 1:1 and 2:1 (host : anion) complexation with divalent phosphate anions. The binding constants between host and anions were determined by means of 1H NMR titrations in CD3CN (HPO42−: log K1:1=6.2, log K2:1=4.9; H2P2O72−: log K1:1=4.4, log K2:1=1.8; AMP2−: log K1:1>7, log K2:1>5) and the affinity of host toward divalent anions, HPO42−, H2P2O72−, and AMP2−, is stronger than that toward monovalent anions, NO3−, BF4−, ClO4−, HSO4−, and PF6−. The CD exciton chirality studies of host with divalent anions, HPO42− and AMP2−, revealed that the two DMAB groups in the 2:1 complexes were arranged with negative chirality (counterclockwise). The dual fluorescence behavior of the DMAB group demonstrated not only the complexation stoichiometry but also the role(s) of the lipophilic countercation such as tetrabutylammonium and/or the hydrophilic residue in AMP during anion recognition.